TSB's 'big bang' IT migration locked 1.9 million customers out of their money for weeks

TSB is a British retail bank, owned since 2015 by Spain's Sabadell Group. For years after the acquisition, TSB's customers still ran on the IT platform of its former parent, Lloyds Banking Group. In 2018, Sabadell decided to move TSB onto its own banking system, Proteo — a migration of about 5.4 million customers and billions of records, attempted in a single 'big bang' cutover over one weekend in April 2018.

The migration went catastrophically wrong. Within hours, huge numbers of customers were locked out of online and mobile banking; the outage stretched into days, then weeks. Worse, some customers could see other customers' account details, including balances and transactions — a serious security and privacy failure. At its peak, about 1.9 million customers were affected, unable to access their money or make payments.

The fallout was severe. TSB's chief executive, Paul Pester, resigned in September 2018. The bank spent hundreds of millions of pounds on compensation, fixing the systems, and lost business, and it was fined by regulators. The episode became a textbook case of how not to run a critical IT migration: a high-risk, all-at-once cutover of a bank's core systems, with inadequate testing and no working fallback, turned a planned upgrade into a weeks-long crisis that locked customers out of their own money.